1. Field of the Invention
This invention relates to sound absorbing or acoustical material and a method of manufacturing same. More particularly it has reference to sound absorbing compositions manufactured from a wet mineral wool pulp that is cast or molded into the form of tile.
Wet pulp molded acoustical tiles are not novel per se. U.S. Pat. Nos. 1,996,033 and 1,769,519 describe compositions and a method of wet forming a molded acoustical tile from a thick aqueous pulp of starch and granular mineral wool. First a starch gel binder is formed by cooking a starch solution and optional fillers and colorants to a thick gel and then granulated mineral wool is added to form a thick wet pulp. The set pulp contains about 80%-95% by volume (about 65%-71% by weight) water, which is necessary to give rise to proper forming properties of the wet pulp and the acoustical nature of the product. During drying, the water is removed resulting in voids that give rise to the highly porous nature and excellent sound absorbing character. The rising cost of energy and the lengthy times involved in drying have led to considerable interest in means to reduce the water required without sacrifice to the porous nature and sound absorbing character of these products. Drying costs and time are proportional to the amount of water removed. Attempts to merely reduce water lead to extremely viscous binder gel difficult to mix properly and insufficient strike-in of the binder into the granulated wool. Previous attempts have bee made to introduce separately generated foams into the wet pulp mix. These efforts so far have failed, as either the foam broke down during mixing into the viscous pulp or the resultant tile was weak and of insufficient dry strength.
The cast products described above utilize a viscous cooked starch solution to impart desired rheological properties and permit proper surface fissure formation upon screeding. The starch is also the binder for the mineral fiber and controls density of the products. In adding a separately generated foam, the main problem is keeping the foam intact after mixing. Previous attempts with numerous stable foams showed the foams very quickly collapsed on attempts to mix with the stiffened pulp. Pulp becomes extremely stiff when prepared with as little a reduction as 20% less water. Chemical additions to liberate gas bubbles generally degraded the starch binder or completely attacked and ruined either the mineral wool or the starch binder. Attempts to in situ foam with conventional surfactants resulted in either total loss of binder strength in the dried board or formed a mono-layer on the wool preventing starch bonding from forming.
Thus it remains an object and advantage of the present invention to reduce the water required for molded acoustical mineral wool products while maintaining desirable rheological properties and, in particular, dry strength. Another object and advantage is to provide a method of manufacturing molded acoustical tile utilizing an about 40% reduction in the water required to be dried from the wet pulp. Still another object and advantage of this invention is to provide a molded acoustical product encountering less edge warpage or bending during drying.